1. Field of the Invention
The present invention relates generally to lighting systems. More particularly, this invention relates to lighting fixtures, or luminaires, or systems containing four or more distinct primary wavelengths of light-emitting devices or groups of devices, e.g., light emitting diodes (LEDs) and systems for additively mixing colors of light to achieve various color matches between the composite light spectrum and a target light spectrum.
2. Description of Related Art
Light sources are varied and well known in the art. Light sources are commonly used to illuminate objects or rooms in the absence of natural light sources. Thus, light sources are very common inside buildings. One application for a light source is theatrical or stage lighting to artificially produce white and colored light for illumination and special effects.
Many conventional light sources produce wavelengths across a relatively broad portion of the visible spectrum of light, for example, incandescent, fluorescent and many high-intensity discharge (HID) lamps. Such light sources may be referred to as white light sources. Other light sources may cover a relatively narrow band of the visible spectrum. Examples of such narrowband light sources include LEDs and lasers, which inherently exhibit a color associated with the dominant wavelength of their spectral power distribution.
Conventional theatrical lighting fixtures typically utilize a lamp that radiates white light, which is then filtered in various ways to produce color when colored light is desired. Filtering subtracts certain wavelengths from a beam with a broad spectral power distribution. For example, the conventional “PAR” fixture includes a white light source (lamp) with a parabolic reflector directing light to a lens with gel color filters, and is typically housed in a cylindrical or can configuration. Conventional theatrical lighting fixtures may be automated with motors that are attached to lenses or to rolls of flexible gels (filters) that move in front of the lamp. Occasionally some fixtures are fitted with multiple overlapping rolls of gels or colored lenses. Using such filters in combination is known as subtractive color mixing and this technique provides a limited range of automated color control. On most fixtures, however, filters are fixed and must be changed manually to alter the color. Manual filter changing can be an expensive and time-consuming process.
It is also well known to combine the light of two different colors to obtain a third color. This is known as additive color mixing. Conventionally, the three most commonly used primary colors—red, green and blue (RGB)—are combined in different proportions to generate a beam that is similar in appearance to many colors across the visible spectrum. Conventional LED lighting fixtures and systems use various combinations of LEDs outputting the primary RGB colors to obtain a desired color of light. There are fixture manufacturers today who utilize a mix of red, green, and blue LEDs to produce color. Typical of such conventional systems are those disclosed in U.S. Pat. Nos. 6,016,038, 6,166,496 and 6,459,919 all to Lys et al. Other conventional LED lighting systems incorporate an additional color, amber, often with the intent of providing means of altering the correlated color temperature (CCT) when the mix of red, green, and blue LEDs is adjusted to produce white light. The general advantages of using LEDs as the basis for lighting fixtures are commonly known by those familiar with the technology in the illumination industry.
A common misunderstanding of human color perception holds that since we distinguish color by using three different kinds of receptor cones in our eyes (a widely understood and proven physiological fact), we therefore perceive only three primary colors of light. The thinking continues toward the mistaken belief that by using a mix of three primary colors of light in various relative intensities, we can precisely duplicate any color in the spectrum.
This conventional, though limited, understanding of human color perception is inaccurate. If it were true that the human eye can only respond to three colors of light, one would be unable to view a rainbow. Instead of a broad wash of graduated colors, one would see only three, very narrow lines of light. One might experience relatively little light radiating from many artificial light sources, such as neon tubes and low- and high-pressure vapor lamps, which produce discrete wavelengths of color that are often not red, green, or blue. The perceived light from other artificial sources would be greatly reduced, since fluorescent tubes (and many other lamps) produce a series of irregular spikes of color along the spectral range, rather than an even mix of all wavelengths.
Other common misunderstandings include the following: the combination of red, green, and blue light is equivalent to “full-spectrum” light; red, green and blue combined in the right proportions can produce true, white light at any CCT that appears and illuminates colored objects in the same way as a real full-spectrum source like midday sunlight; an increase or decrease in amber light alone is sufficient to alter the CCT of a white-light mix across a broad range of CCT values.
LED-based lighting fixtures that implement any of these misconceptions produce light that is inadequate for a broad range of effective, primary illumination. RGB fixtures produce colored light with relatively poor saturation across the spectrum, except at red, green, and blue. RGB fixtures illuminate colored objects in an unnatural way, making many colors appear hyper-real or more vivid than under midday sunlight but also making them appear less differentiated from one another, with a strong tendency to make colored objects appear either redder, greener, or bluer than normal. RGB fixtures exhibit relative luminance levels that are difficult for an average user to predict when mixing colors, because they do not correlate with the relative luminance levels of conventional lamps with filters of similar colors. White light from RGB fixtures appears weak, empty, or grayish to many observers. RGB fixtures often produce an undesirable response on human skin tones, making many flesh colors appear ruddy or slightly greenish or grayish. RGB fixtures have a limited range of CCT values that appear rich, full, and satisfying to the average observer.
The addition of amber to an RGB fixture (RGBA) for the purpose of “color correcting” or lowering the CCT of its white light often results in light that appears unnaturally pinkish. Most such four-color, RGBA, lighting systems do not contain amber LEDs that together produce a high enough level of relative luminance to significantly add to color-mixing capabilities or to alter the undesirable rendering of colored objects and skin tones.
Prior art by Cunningham, U.S. Pat. No. 6,683,423, describes a lighting apparatus having groups of distinct light-emitting devices, e.g. LEDs, that can be controlled to produce a beam of light having a spectrum that closely emulates that of any one of a number of conventional light sources, e.g. an incandescent bulb, and that has a normalized mean deviation (NMD) across the visible spectrum, relative to that of the beam of light being emulated, of less than about 30%.
There are flaws in the approach taken by Cunningham to describe the output of the claimed invention. The standard of 30% or less NMD does not correlate with the human eye response. The invention could achieve 30% NMD—or even much less—and still produce a light beam that behaves differently on illuminated objects and that appears very different to the average human observer than the one being emulated. Cunningham provides no metrics for relating the output of this invention to the response of an average human observer, which is the most critical component of measurement when describing an apparatus suitable for use as part of a lighting fixture. Without such metrics the invention is too broadly defined to be of real value.
For example, if the invention produces a spectral distribution curve that is slightly above the reference at wavelengths shorter than 550 and slightly below the reference at wavelengths longer than 550, the composite beam would have a much more dominant blue component than the one being emulated, although the NMD for the entire spectrum might be well within 30%. Not only would this make the beam itself have a different apparent color or whiteness, it would alter the way the beam illuminates colored objects, perhaps drastically.
In another example, if the majority of the spectrum of the invention is closely related to the spectrum of the reference source, the invention could completely omit a portion of the spectrum—a gap perhaps as large as 70 to 80 nm wide—and still have a normalized mean deviation that is relatively low. Again, this could produce drastic apparent differences to the average human observer, both in beam color or whiteness and in the illumination of colored objects.
In a third example, the Cunningham invention could produce a spectrum that was nearly identical to the reference in all but a very narrow range of wavelengths—perhaps a range only 5 nm wide. In that 5-nm range, the invention could produce a huge spike in spectral output, equivalent to the addition of a very bright, deeply saturated colored light, and still produce an NMD for the whole spectrum that is well under 30%. Obviously, the resulting light would look nothing like the reference, nor would it illuminate colored objects in the same way.
Accordingly, there exists a need in the art for LED arrays, lighting fixtures and systems that not only include LEDs emitting conventional RGB or RGBA colors, but that emit other colors as well. There also exists a need to define these inventions by parameters that are based on the human visual response, in order to provide a more certain guarantee that the inventions produce light that is desirable for a broad range of applications. Such LED arrays would overcome the inherent limitations of all known lighting fixtures that include multiple colors of LEDs.